Processes to prepare isocyanates by reacting primary amines with phosgene in an inert solvent are known. When the primary amines are aromatic amines, the aromatic amine can be converted with comparative ease to a high purity aromatic isocyanate by contacting the phosgene directly with the aromatic amine in a solvent. In the case of aliphatic amines, the overall reaction with phosgene is relatively slow in comparison, thus undesirable chloroderivative by-products are often formed.
For these reasons, the production of aliphatic isocyanates is usually accomplished in one of two ways. The first method, known as the salt method, involves converting the amine to the hydrochloride or the carbonate. The resulting salt is transformed into the isocyanate by passing phosgene through a solvent/hydrochloride slurry at elevated temperatures. The second method, known as cold-hot base approach, comprises adding the free amine to excess phosgene, preferably condensed in an inert solvent at low temperatures. The resulting slurry is heated while more phosgene is passed through to complete the reaction. Aliphatic diisocyanates have been manufactured using both methods at temperatures ranging from 130 to 175° C. in a variety of inert solvents. At these temperatures, considerable amounts of chloromonoisocyanates are noted to form. Unfortunately, the boiling points of the desired diisocyanates and the chloride impurity derived from them are often found to be quite close, thus rendering separation by distillation difficult.
When aliphatic isocyanates containing the chlorinated impurity are used to prepare polyurethane polymers, the chlorinated impurity is known to adversely affect the desired urethane forming reaction. Consequently, a large amount of work has been compiled detailing process options to reduce the chlorinated impurities obtained during the phosgenation step. GB1086782 describes a process for the phosgenation of aliphatic amines, or their hydrochloride equivalents, at temperatures of 120 to 180° C. in a weight ration of solvent/amine of 18:1 to 30:1. GB1050555 discloses a process for preparing an organic isocyanate via phosgenation of a primary amine hydrochloride containing a surface active substance in an inert solvent. JP55-88451A describes the use of a continuous high temperature phosgenation process. U.S. Pat. No. 2,642,449 discloses a process utilizing phosgene injected into the reaction mixture at elevated pressure.
Even when the desired reactions dominate, additional problems are incurred due to the inherent reactivity of aliphatic amines. When either the salt or cold-hot base process is employed, a carbamoyl chloride is formed as an intermediate. The carbamoyl chloride is then subjected to dehydrochlorination so that the aliphatic polyisocyanate is prepared. The dehydrochlorination of the carbamoyl chloride into the polyisocyanate takes place at a low reaction velocity and in general, requires a high reaction temperature of at least 120° C., usually 130° C. or higher. When the polyisocyanate so formed is exposed to heat for a long time, the polyisocyanate tends to become tarry, resulting in a reduction in the production yield. Further, hydrochloric acid gas which has been formed as a result of decomposition of the carbamoyl chloride reacts the resultant polyisocyanate, whereby the carbamoyl chloride is formed again. The carbamoyl chloride becomes tarry at a far higher rate than the polyisocyanate, so that the yield is reduced further.
The processes of the prior art, however, have been largely ineffective when used with amines that have a high degree of symmetry or crystallinity, and isomeric mixtures thereof. Amines of this type generally yield products having high levels of oligomeric and/or intractable content during the phosgenation reaction.
The prior processes have also been cost ineffective in that they require large amounts of solvent, thus incurring extensive distillation and recycling costs. As a result, the conventional processes are not amenable to the commercial production of isocyanates directly from crystalline or ordered amines.
It is an object of the present invention to provide an economical process for preparing an aliphatic or cycloaliphatic isocyanate, more specifically isocyanates based isomeric mixtures containing high levels of a symmetrical or crystalline isomer, in high yields by the phosgenation process.